DEAR CAROLYN: My boyfriend and I are in our early 20s, and have been together for about 2 1/2 years. In recent months we've talked of "being together for the long haul."

The problem is this: While I see the value in very healthy eating and daily exercise, he isn't quite on the same page. He is the product of an almost unbelievable gene pool (can't gain a pound, blemish free, etc.) and sees exercise as a time-consumer. He often feels sick or anxious for long periods of time, but when I suggest exercise as a remedy he disengages.

I'm starting to worry, as these episodes are becoming more frequent. How can I convey to him that my desire for him to exercise and eat right doesn't come from a place of criticism or unhappiness with his appearance, but rather from a health standpoint? I love him dearly, but I can't stand by and watch him become the picture of heart disease or Type II diabetes. How can I get him to understand that genetics might guarantee a great appearance but not a truly healthy person?

— Heart Weary

Dear Heart Weary: You don't know his genes are perfect. You don't know his cheesy-poof habit will damage his health. You don't know whether habits will govern your lifespan or a falling piano will.

As for the "long haul," that certainty is for suckers. You don't know how anything will be, you know only what is.

In your letter, there are only two certainties, and they need the attention you're currently misdirecting to his diet and exercise habits.

The first is that he is "sick or anxious" — "often" — and unwilling to deal with that head-on. This is as serious as problems get in relationships.

Why? Because once you recognize there's no such thing as certainty when it comes to futures, you start to connect some dots. First dot: Anything can happen. Second dot: When bad things happen, you can handle them well or badly. Third dot: Handling them well will dramatically improve your quality of life. Fourth dot: "He disengages" equals not handling things well. Fifth dot: The "long haul" with someone who doesn't handle difficulties well promises to be tough.

So please stop talking about his diet and exercise — apologize for it, in fact — and start talking about talking. Unless you want to be the picture of dysfunction, you both need to prioritize transparency on whatever topic life spits out.

Which brings us to the second certainty: Any problem in any relationship has two parts, yours and the other person's, and you can't solve problems by trying to fix the other person's contribution. Starting a sentence, "How can I get someone to ____," says you don't understand this basic boundary.

You are you, he is he, and what you create together is the result of choices willingly made, each on your own side of the line. If his habits bother you, then it's your prerogative to say so. To change them or not is his prerogative. To respond to his choice — by accepting him as-is or breaking up — is your prerogative. See?

The good news is, addressing the second certainty is likely to help with the first: People tend to engage more willingly with those who both draw and respect these lines.